This robot is a single-RCX biped with the ability to turn within its own footprint.
It's also my first foray into doing a 'tough-looking' robot, as opposed to one that
is purely functional.

Turning works by advancing the robot until the leg that leads the turn is in front
(e.g., the right leg is forward for a right turn). To make a right turn, the robot
takes its weight on the right foot (left leg off the ground) and turns the right
ankle turntable about 15° to the right; it then takes half a step back onto the
left foot, raising the right foot so that it can straighten the right ankle. It
then moves its weight forward onto the right foot again, turns the ankle another
15°, and so on. This procedure can be repeated until the robot has turned the
desired amount. The advantage of turning in this way is that the robot turns
pretty much within its own footprint -- no requirement for maneuvering in
large-diameter circles.

MECHANICS

The lower (belly) motor drives a worm-24t gearbox. The axle from the 24t gear
connects to two cams, one for each leg, offset 180°, lifting and moving the legs
alternately to provide a walking action.

The two motors mounted high on the rear supply power to rotate the large
Technic turntables which form the ankles; one motor for each ankle. The power is
transferred to the ankles via Znap flexible axles; for each leg, one link runs from the motor
to a small double-bevel gear at the 'knee' and drives a medium double-bevel gear
which is flexi-linked to the worm screw at the back of the 'ankle'. Having heavy
legs is really the death of chicken-walker robots like Biped_II because the legs
tear off after 10 or so paces -- the Znap flexy-drive links allowed me to put all
the weight in a well-braced position at the center of the robot, rather than having
the weight of the motors down at the ankles.

The position of the moveable elements is monitored by three rotation sensors:
one each for the ankles and one for the drive motor. The ankle sensors each have
a 24t gear which is driven by an 8t gear on the same axle as the worm gear that
turns the ankle turntable. The drive motor sensor has a pair of 12t gears driven
by the 24t gear that is connected to the walking cams.

I tried to make the legs as light and stiff as possible -- the other problem with
'chicken-walkers' is that the legs tend to sag inwards so that, as the rear foot
comes forward, it catches on the heel of the foot in front. The lower part of the
leg is mostly composed of single-bend liftarms pinned together whilst the 'thighs'
are Technic bricks, cross-braced with plates to make box-section girders.
The ankles are large Technic turntables; the feet are Technic bricks, cross-braced
with double-bend liftarms and stiffened with 2-stud wide plates at the trailing
edge of the heel. These plates also have the effect of tilting the whole robot
slightly forward which improves the balance and makes it look more 'poised for action.'

PROGRAM

The robot runs a simple NQC program that interprets messages from the LEGO remote control:

message 1 starts a turn to the left;

message 2 toggles forward motion on and off (i.e., the first message 2 starts
the robot walking in a straight line, the next message 2 makes the robot stop,
the next message 2 starts the bot walking in a straight line again, and so on);

message 3 starts a turn to the right.

Check the code for more details. [I must admit, I had fun making the robot as menacing-looking as I could. Hope you like it.]